They are stuck in the season’s doldrums. As short as the days are, they can seem to be very long. The playoffs are many pages away on the calendar. The tedium of the season has shown in the Bruins’ play.

The first line wasn’t putting away its chances. Brad Marchand, who’s been puzzling his bosses with his peaks-and-valleys play, could not be trusted from shift to shift. Loui Eriksson was trying to find his projected trajectory upon returning from his second concussion. Gregory Campbell’s inconsistency at the dot was making the fourth line chase the game instead of controlling its pace.

Thursday’s 4-2 win over Dallas at American Airlines Center marked one step in correcting some of those issues. For the Bruins, losers of three of their last four entering the night, it was as close to a must-win as they’ve had all season.

The four-line strategy is crucial to the club’s success. The Bruins play a system that is demanding on their forwards. To help out their defensemen, the forwards are instructed to come back hard for the puck. Once they have it, they’re supposed to accelerate through the neutral zone and reach full flight once they cross the offensive blue line.

It is physically and mentally tiring. Unlike other teams, the Bruins cannot afford to lean hard on their top three lines. If they did, their legs and lungs would fade by the third period.

Against the Stars, the Bruins showed how efficient they can be, especially late in games, when their four-line attack is howling at full song.

It was no coincidence that their third period was their best. Marchand snapped a 1-1 tie with the first of three third-period goals. Milan Lucic, replacing Zdeno Chara as the net-front hulk on the No. 1 power-play unit, redirected a David Krejci shot past Kari Lehtonen for the winning goal. Patrice Bergeron, the catalyst behind Marchand’s goal (his forecheck caused Erik Cole to give up the puck), put an end to Dallas’s rally with an empty-netter.

The goals marked breakthroughs for the first two lines. Marchand was coming off a 1-1—2 performance in Tuesday’s 4-3 loss to Toronto. Julien used Marchand for 20:01 of ice time, his second-highest total of the season.

On Thursday, Marchand ripped off five shots. In the first period, Marchand tussled with Trevor Daley. His third-period strike gave him goals in consecutive games for only the second time this season. Inconsistency has been Marchand’s biggest blemish. Now, the coaches have a better idea of how Marchand will perform from shift to shift. He is regaining their trust.

Consistency hasn’t been a problem with the No. 1 line. Finishing has been another story.

Lucic hadn’t potted a goal since Dec. 21. He had scored just once in 18 games. Krejci hadn’t scored since Dec. 31.

Penalty trouble took away some of their rhythm. Dallas had six power plays. Only Krejci takes regular shorthanded shifts. But they made the most of their looks. At 19:56 of the second period, Krejci scored a timely goal to tie the game at 1-1. Lucic started the play by retrieving the puck in the corner, then wasted no time dishing it back out front. Krejci did the right thing by finding the soft spot in the Stars’ coverage.

In the third, the combination clicked again, this time in reverse.

“We’ve been close,” Lucic said. “Especially in the Toronto game, we had chances that we didn’t bury. Sometimes that’s going to happen. You want to try to get out of the slump, especially as a line, as quick as possible. Good to have it here tonight.

“If you look at our game, we didn’t spend much time at all in our defensive zone. I think that’s why we were able to get some results.”

The Bruins’ two best forwards didn’t land on the scoresheet. Carl Soderberg was active in the offensive zone. Soderberg skated well, mucked in the corners, and was quick to put pucks on goal. Soderberg landed five shots on Lehtonen. Soderberg’s creativity and strength on the puck makes the No. 3 line dangerous in the offensive zone.

Campbell turned in one of his best all-around games. In the first period, he appeared to score his second goal in as many games. The call was overturned, however, when video review determined that Campbell kicked the puck past Lehtonen with his right leg.

Campbell shrugged off the dismissed goal. He and Daniel Paille used their speed and feistiness to back up the Dallas defenders. The pair had been misfiring on the penalty kill, where they had been overcommitting and leaving seams for opponents to pick apart. Campbell and Paille were in better control against Dallas’s power play. At the dot, Campbell won 9 of 12 faceoffs.

The Bruins need their four-line rhythm to continue. They have their first rematch against Chicago on Sunday at the United Center. The following day, they host the Kings, who handed them a 4-2 loss Jan. 9. Following a four-day pause — their longest remaining in-season rest — the Bruins have a two-game road trip against the Flyers and Islanders, two of the East’s hottest teams.

The Bruins will reenergize soon. The Olympic break is in less than a month. Once play resumes Feb. 26, there will only be a week until the March 5 trade deadline. The season that seems endless now will not have many days left by then.